Method of treating variable transition temperature alloys

ABSTRACT

METAL ALLOYS OF THE FORMULA TINIXCO1-X, TICOXFE1-X, WHEREIN X IS AN INTEGER FROM 0 TO 1 UNDERGO MARTENSITIC TRANSITIONS OVER A VERY WIDE TEMPERATURE RANGE FROM LESS THAN 4*K. TO 1000*K. THE METAL ALLOYS ARE WORKED AND HEAT TREATED TO EFFECT A REVERSION BACK TO AN ORIGINAL CONFIGURATION AS THE RESULT OF THE MARTENSITIC TRANSITION OF THE ALLOYS DUE TO THE HEAT TREATMENT.

United States Patent US. Cl. 148-115 2 Claims ABSTRACT OF THE DISCLOSURE Metal alloys of the formulas TiNi Co TiCo Fe wherein x is an integer from 0 to;1 undergo martensitic transitions over a very wide temperature range from less than 4 K. to 1000" K. The metal alloys are worked and heat treated to effect a reversion back to an original configuration as the result of the martensitic transition of the alloys due to the heat treatment.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-impart of application Ser. No. 579,185, filed Sept. 9, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to structural alloys and more particularly to alloys of an intermetallic compound type characterized by a capability of effecting a martensitic (ditfusionless) transition at a predetermined temperature.

A considerable amount of effort has been expended heretofore in an attempt to develop a series of alloys having the same unique properties as the near stoichiometric TiNi (disclosed in US. Pat. No. 3,174,851) where TiNi denotes an alloy containing near equiatomic quantities of titanium and nickel, sometimes referred to as Nitinol, and especially to develop alloys which exhibit a unique martensitic (diffusionless) transition at any selected temperature throughout a desired temperature range Without effecting any of the desirable attendant features of the stoichiometer TiNi. Attempts to provide such a series of alloys have heretofore met with little success. probably because of a lack of a clear understanding of the atomic structural transition associated with the unusual mechanical and physical properties found ni TiNi.

The TiNi-base alloys, particularly those near stoichiometric TiNi composition, are capable of performing unique and heretofore unknown property feats. These unusual propeties of mechanical memory, acoustic damping, energy conversion, etc., have opened an entirely new area of potential applications. Currently such applications as self-erecting aerospace and hydrospace components, low acoustic noise machine mountings and devices,

-prestressed bindings and reinforcements, stored energy tools, energy converters, temperature actuated devices (switches, fire alarms, etc.), clock mechanisms, cryogenic components, are but a very few of the possible uses.

Near TiNi alloys, while efficient in the performance of the above tasks, are limited by a somewhat narrow critical temperature range. Therefore if one wanted to use materials in such a way as to fully utilize the martensitic transition properties of alloys-one was severely limited by the temperature range of this transition. Thus, if one wished to erect a structure on the ocean floor, where the water temperature is around 271 K., one could not use a TiNi alloy unless one were willing to continually heat the alloy to maintain its transition temperature of around 439 K. so that the structure would maintain its stiffness.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a series of intermetallic alloys especially exhibiting the unique properties of a mechanical memory, acoustic damping, direct conversion of heat to mechanical energy, and useful mechanical properties, e.g., strength, toughness, etc.

Another object is to provide a series of alloys having similar properties yet each alloy exhibiting anindividual martensitic transformation property at a diverse predetermined point in a desired temperature range. a

A further object is to provide alloys which exhibit a martensitic transition characteristic over an extended temperature range.

Another object is to provide alloys which make use of relatively inexpensive elements and which are readily available for engineering structures.

These and other objects are attained by the formulation of alloys that can undergo unique atomic structural changes at any predetermined temperature from virtually zero degrees Kelvin (273 C.) to approximately 1000 Kelvin (727 C.) said alloys represented by the following structural formulas:

- martensitic transition temperature range C.) of TiNi as a function of the atomic (and weight) percent of Ni;

FIG. 2 is a graphical representation of the logarithm of the unique martensitic temperature K.) as a function of the free electron concentration of the ternary alloys TIiNi Co and TiCo Fe and FIG, 3 is a graphical representation of the unique martensitic transition temperature K.) as a function of the free electron concentration of TiNi Co and TiCo Fe DESCRIPTION OF THE PREFERRED EMBODIMENTS The unique martensitic transition which the alloys of the instant invention undergo is effected merely by causing the alloys to be deformed at a temperature below their critical temperature (which is hereinafter defined) and thereafter heating these alloys to or above their critical temperature. Thus, these alloys will, when heated to their critical temperature, revert back to a predetermined or original configuration. This original configuration will be the configuration of the alloy as it is cooled down below its critical temperature. For example one can cast a straight rod of the alloy, cool it down below the critical temperature, deform it and then heat it to the critical temperature whereupon the rod will revert back to its original configuration (i.e., a straight rod). Similarly a wire can be made so that it is curled, the temperature can be lowered below the critical temperature, the wire can be straightened out and then heated to the critical temperature whereupon the original curled configuration is returned to the wire.

The term critical temperature as hereinbefore and hereinafter used, refers to the temperature at which the alloy under consideration exhibits a martensitic (diffusionless) transition. Near stoichiometric TiNi and the related alloys of this invention exhibit unusual overt properties as a function of deformation and temperature. The near TiNi alloys are capableof transforming heat energy directly and efficiently into mechanical energy. The near TiNi composition composition alloys have a latent heat near -6 calories per gram at the critical temperature. Also acoustic damping, and both elastic and mechanical properties (particularly yield strength) change drastically in the'temperature range covered by the critical temperature. The critical temperature associated with the atomic structure transition varies drastically with minor variations in composition. The alloys recorded show the temperature maximum of 439 K. (166 C.) for stoichiometric TiNi and falling to well below 273 K. (0 C.) for slightly Ni-rich'variations on the TiNi composition as shown in the graph of FIG. 1, wherein the martensitic transition temperature range of TiNi is shown plotted as a function ofthe atomic percent composition of Ni (remainder essential Ti) and also plotted with the corresponding weight per cent composition thereof on the same axis. 7

While this temperature range of'martensitic transition is impressive and useful based upon normal ambient temperature conditions, even exceedingly minor amounts of excess Ni in these binary alloys tend to cause the formation of a second phase of TiNi A very minor quantity of the second phase TiNi like the normal nonmetallic impurities Ti Ni O, Ti Ni NTiC, etc., can be tolerated, but these insoluble impurities tend to reduce the efficiency of TiNi to perform in a manner as stated above.

A desire to provide alloys with the unique overt properties of near stoichiometric TiNi alloys at any desired temperature from essentially absolute zero (0 K.) up to and in excess of 439 K. (166 C.) is accomplished without damaging the efficiency of the overt properties required by the careful and thoughtful alloying with other elements as shown in Table I.

4 FIG. 2. These graphical data, on the alloys formed from the elements of the first long period when combined in the proper ratio, immediately indicate that progressively lower critical temperatures are possible by increased substitution of Co for Ni (TiNigCo on an atomforatom basis. This substitution does not in' any way alter the Ti content of the-ternary alloysof this series..

All ternary phases, TiNi, Co and TiCo l=e formed by replacing Ni with Co, and Co with Fe, have crystal structuressimilar to the basic binary- TiNi compound. The crystal structure of TiNi has been determined through single crystal and powder X-ray defraction studies and the existence of a martensitic transition (without a first order crystallographic transformation) has been confirmed. Assuming the number of free electrons contributed by Ti, Fe, Coand Ni in their alloys to be 4, 8, 9 and 10 (number of electrons beyond the closed shell) respectively, the transition temperature, plotted as a function of the free? electron concentration (number of free electrons per atom) in these alloys, is a smooth curve as shown in FIG. 3. This curve indicates that the martensitic type transition observed in TiNi is not unique to this compound alone, but is prevalent also in those binary compounds of TiCo and TiFe and in the ternary combinations of these elements as stated above.

In the case of the ternary alloys formed from Ti with selective variations of Ni, Co and Fe it is possible to attain any desired critical temperature from virtually zero degrees Kelvin to 439 Kelvin. Selected ternary combinations of Ti, Ni, Co have been compounded, in the ratio of TiNi Co and processed into test specimens. These ternary systems have exhibited all the same unique overt properties asTiNi (critical temperature 439 Kelvin) but at a lower temperature dependent upon the amount of Co substituted for Ni.

Further observing Table I, it becomes apparent that the intermetallic compounds ZrRu, ZrRh and ZrPd should TABLE I.DATA ON ELEMENTS AND INTERMETALLIC ICOMPOUND PRODUCTS FORMED FROM THESE ELEMEN S Electron critical Atomic Atomic concentransition radius, radius Valence Compound Crystal tration, temper- A. ratio 1 electrons formed structure e./a. ature K.

1st Long Period (Periodic Table) 4 8 TiFe Complex 6.0 4 9 TlCo Complex 6. 5 35 40 'IiN i Complex 7. 0 439 2116 Long Period (Periodic Table) 1. 59 4 1. 32 1. 20 6 ZrRu Complex 6.0 40 1.35 1.18 9 ZrRh do. 6.5 653 1. 37 1. 16 10 ZrPd .do. 7. 0 1,000

3rd Long Period (Periodic Table) 1. 56 4 1.33 1.17 8 HfOs Complex- 6.0 1.35 1.16 9 HfIr d0. 6.5 1.39 1.12 10 HfPt do. 7.0

Ti( Radius) Zr( Radius) 1 Radio obtained in each period by dividing Fe or au=9A.; 3sub=2-98 A.

5 To be determined.

Observing-the data given in Table I, certain basic structural similarities are evident. These similarities exist mainly in the atomic radius ratios and valance electrons of the corresponding metallic elements and the crystal structures and electron concentrations of their intermetallic compound products. In this table consideration has been given to the first, second and third long periods of the Atomic Periodic Table. Plotting the natural logarithm of the transition temperatures (critical temperatures) for the first long period compounds, TiNi, TiCo and TiFe as well as intermediate ternary alloy compositions, e.g., TiNi Co and TiCo Fe as a function of free electron concen- Co or Ni(Radius) Ru or Rh or Pd(Radius)' behave in a similar manner. Also the compounds of HfOs, HfIr and HfPt of the third long period are candidates for the same atomic structural behavior. Experimental evidence has confirmed these latter two series compounds also to have critical temperatures and exhibit the same unique behavior as was found in the TiFe-FiCo-TiNi series. Further, ternary substitution, e.g., ZrPd Rh and HfPt -Ir ,'etc., should be permissible to yield interme diate critical temperatures. As the table indicates, the compounds of 'Zr, Hf and their intermediate ternary alloys provide higher critical temperatures for the. atomic structural transition-to occur. As a result, from a practical nation, a smooth linear curve is obtained as shown in standpoint, the critical temperature has been extended from absolute zero (zero degrees K.) to approximately 1000 K. (717 C.).

Both the yield strength and the elastic modulus (Youngs modulus) increase significantly above the critical temperature. To take advantage of the increased stifiness above the critical temperature it is necessary to tailor alloys that have critical temperatures below the operating environment range for a particular application. This would be particularly significant in self-erectable structures for both hydrospace and aerospace applications. In order that structures may self-erect, anywhere, such for example as in sea-water, it is necessary to provide self-erecting alloys with critical temperatures well below the minimum sea water temperature of minus 2 C. (271 K). To accomplish this and to provide efiicient mechanical memory, etc., cobalt substituted alloys (TiNi Co could be invalnble.

Obviously, numerous modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that Within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A method of effecting a reversion back to an original configuration by means of a martensitic transition of a metal alloy at a predetermined critical temperature from about 35 K. to about 439 K., said alloy having the formula TiNi,,Co wherein Ti denotes titanium and constitutes approximately 50 atomic percent of the composition, and the term Ni Co denotes nickel and cobalt respectively and make up the remaining approximately 50 atomic percent of the composition, and

x is a factor which varies from greater than to less than 1 whereby the relative percentage of nickel and cobalt varies inversely from less than 100 percent to more than 0 percent with respect to each other on an atom for atom exchange basis as the martensitic transformation temperature varies correspondingly from about 439 K. to about 35 K. in a nonlinear manner which comprises the steps of:

(1) subjecting said alloy while in said original configuration to deformation at a temperat re below wherein Ti denotes titanium and constitutes approximately 50 atomic percent of the composition, and the C0 Fe term denotes cobalt and iron respectively and make up the remaining approximately 50 atomic percent of the composition, and

x is a factor which varies from 1 to 0 whereby the relative percentage of cobalt and iron varies inversely from 100 to 0 percent with respect to each other on an atom for atom exchange basis as the martensitic transformation temperature varies correspondingly from about 35 K. to about 4 K. in a nonlinear manner which comprises the steps of:

(1) subjecting said alloy while in said original configuration to deformation at a temperature below the critical temperature to change the shape thereof and (2) thereafter heating said alloy to or above said critical temperature to elfect said reversion back to said original configuration.

References Cited UNITED STATES PATENTS 4/1920 Schcuch 75170 3/1965 Buchler et al. 75-470 OTHER REFERENCES CHARLES N. LOVELL, Primary Examiner US. 01. X.R. 

